It is common practice to equip apparatus such as a pressure vessel, especially a boiler and particularly a marine-propulsion boiler, with a safety valve or pressure relief valve which is adapted to vent the vessel when the pressure therein exceeds a predetermined limit. In other words, vessels which are to be maintained under pressure can commonly provided with pressure-relief valves intended to avoid over-pressurization of the vessel and danger to operating personnel and to the structure.
The earliest and most rudimentary systems of this type provide the safety valve with a valve member which is urged against the seat by a spring of controlled force to seal the interior of the vessel as long as the spring force is greater than the force of fluid pressure acting in the opposite direction upon the valve member.
When the pressure in the interior of the vessels exceeds a predetermined limit, the valve member is displaced from its seat to vent the interior of the vessel.
This system has the disadvantage that the valve member vibrates or impacts against its seat as soon as the pressure drops sharply as a result of venting. Furthermore, the valve never opens fully because the forces readily assume a balance between the spring and the fluid pressure force, particularly when the valve is cracked to permit partial release of pressure. As a result, the vented fluid, flowing at high velocity through the constriction provided between the valve member and the seat, rapidly erodes both the valve member and the seat.
As a result of the impact opening and closing of the valve member, the valve is noisy and requires frequent maintenance. The spring, which is located on the boiler itself, must be repeatedly adjusted at an inconvenient and dangerous location and frequently changes in its characteristics.
A solution to the problem has been proposed as will be apparent from my and the earlier patents referred to above. In this system, the safety valve is provided with a piston which is slidable in a cylinder while the cylinder is connected to a pressure-detecting system.
The pressure-detecting system can include one or more control valves and means for actuating the same to pressurize the cylinder of the safety-valve system or to vent the cylinder to the atmosphere, thereby allowing complete blockage of the valve or complete opening thereof in a uniform manner.